The present invention is directed to a method and apparatus for generating a plasma in a gas, and more particularly, the invention is directed to a method and apparatus for generating a plasma in a gas at atmospheric pressure using a thermal source and an electrical source.
During the treatment of silicon wafers used in integrated circuits, a plasma is generated in a gas during several different production steps, including etching, ashing, wafer cleaning, and chemical vapor deposition.
During integrated circuit fabrication, the starting material is typically a silicon substrate covered on one side by a dielectric, or insulating film. A desired circuit pattern for a given layer of the integrated circuit is etched into the dielectric film covering the silicon substrate. To accomplish this, a masking material is disposed on the area of the dielectric film where etching is not desired. In other words, the material masks all areas of the insulting film that will remain, and what is not masked, defines the circuit pattern. Photoresist is the most common masking material. The photoresist must have good adherence to the dielectric film, good coating integrity, and the ability to substantially resist attack from the etchant. First, an etching gas is selected so as to generate active species which are chemically reactive with the dielectric material, but substantially less chemically reactive with the photoresist. The etching gas is generated in a plasma, a highly ionized gas composed of an approximately equal number of positive and negative charges, as well as other non-ion particles. The plasma is typically generated by electric current, radio frequency or microwave energy. The plasma is then supplied to the surface of the dielectric being etched, causing the reactive species of the etching gas plasma to diffuse into the surface of the dielectric film. The etching gas reactive species chemically react with the dielectric film to form a volatile by-product. The volatile by-product is then desorbed from the surface of the dielectric film and diffuses into the bulk of the etching gas.
After the pattern is etched into the dielectric layer, the photoresist that was used to define the metal circuit pattern on the dielectric layer is removed. Also, any post-etch residues including side wall polymer deposition also must be thoroughly removed or stripped from the underlying dielectric layer. There are two generally recognized stripping processes for removing photoresist and post etch residues remaining on the surface after the dielectric etch is complete. The residual photoresist and post etch residues can be removed by using either a wet or a dry chemistry process. Wet chemistry involves removing photoresist and post etch residues by dissolution in a suitable acid or solvent. However, the prohibitive cost of wet chemistry, environmental concerns associated with its use and disposal, and product contamination issues have led most manufacturers to use a dry process.
One dry chemistry process used to strip photoresist and photoresist residues from the dielectric layer is commonly referred to as ashing. The process of ashing is similar to the etching process. Ashing is a technique by which the residual photoresist and post etch residues are exposed to a plasma. Typically, the plasma is generated from a gas mixture containing oxygen gas as one of its components. The highly reactive oxygen plasma reacts with or oxidizes the organic photoresist layer. The oxidation or combustion products resulting from the ashing operation are volatile components such as carbon dioxide and water vapor, and are carried away in a gas stream. Ashing is preferred to wet chemical removal because fewer process steps are involved, less handling of the substrates is required, the number of chemicals used is smaller and required chemical handling equipment are less complex, and ashing is more environmentally acceptable. Finally, after the ashing process is complete, the etched pattern within the dielectric film layer is filled with copper or other conductive material. The entire process can then be repeated to form multi-layer integrated circuits.
In integrated circuit production, thin films are utilized for a host of different applications. One advantageous method of depositing thin films is Chemical Vapor Deposition, or CVD. CVD is defined as the formation of a non-volatile solid film on a substrate by the reaction of vapor phase chemicals. CVD is often preferred over other deposition methods because it can achieve high purity deposits, a great variety of chemical compositions can be deposited, and good economy and process control is obtainable. A basic CVD process starts with reactant gases and diluent inert gases introduced into a reaction chamber. The gas species move to the substrate and the reactants are adsorbed on the substrate surface. Next, the reactants undergo migration and film-forming chemical reactions. Finally, the gaseous by-products are desorbed into the gas stream and removed from the reaction chamber. The energy to drive a typical CVD reaction is most commonly thermal. Plasma Enhanced Chemical Vapor Deposition, or PECVD, uses not only thermal energy, but also an rf-induced glow discharge to transfer energy into the reactant gases.
Several basic plasma generating methods are common in the prior art. U.S. Pat. No. 5,330,578 to Sakama et al. discloses a plasma gaseous reaction apparatus including a reaction chamber, a system for supplying reaction gas to the reaction chamber, and a pair of facing electrodes disposed in the reaction chamber. The reaction chamber also includes a pressure control system for adjusting pressure to a predetermined value, and a vacuum pump for exhausting the chamber. The plasma is generated by supplying high frequency power to the electrodes from a power source. A high frequency signal source (13.56 MHz) is used for the power source. When the power is supplied, a plasma is generated between the electrodes. A plurality of substrates are simultaneously treated within the reaction chamber.
Plasma generating methods at atmospheric pressure using metal electrodes as cathodes are also known in the prior art. These methods often required a high frequency signal with a voltage as high as 20 kV to maintain a plasma generated between two electrodes. However, in these techniques, the metal on the electrodes tends to breaks down over time and cause wafer contamination. The present invention seeks to use a flame to initiate and sustain a DC, RF or microwave plasma to avoid electrode breakdown and wafer contamination. Further, the voltage required to maintain the plasma may be lower than 1 kV, significantly reducing energy costs.
A need exists for a lower cost, flexible plasma generation technique with applications to etching, ashing, wafer cleaning and chemical vapor deposition processes. The present invention advantageously allows for a highly flexible plasma generation, with high chemical species disassociation and lower energy costs, while minimizing wafer-contamination from electrode breakdown.
The present invention addresses the need to generate a plasma in a process gas at atmospheric pressure using an inexpensive, flexible method and apparatus with applications to etching, ashing, wafer cleaning and chemical vapor deposition processes.
The method practiced in accordance with an exemplary embodiment of the invention, flows a process gas into a reaction zone, heats the process gas within the reaction zone with a heat source, passes an electric current through the process gas in the reaction zone to generate a plasma within the process gas, and further positions a substrate in the path of the reaction zone output for treatment with the plasma. The invention provides efficient generation of a plasma, with higher proportions of disassociated molecules within the gas and requires lower voltages to maintain the plasma. This results in decreased energy costs, while minimizing wafer-contamination from electrode breakdown.
The method may use a burner as a heat source, and charged electrodes, a microwave cavity, or RF coils to generate the current within the plasma. Practice of the invention allows creation of a reaction zone at atmospheric pressure for treating a silicon wafer. The wafer treatment process may be part of commercial production etching, ashing, wafer cleaning, or chemical vapor disposition process.
These and other objects, advantages and features of the invention will become better understood by review of the accompanying detailed description of a preferred embodiment of the invention which is described in conjunction with the accompany drawings.